A potential mediator of collagenous block copolymer gradients in mussel byssal threads

Mussel byssal threads contain unusual block copolymer-like proteins that combine collagen with flanking domains that resemble silk-fibroin (preCol-D) or elastin (preCol-P). These are distributed in complementary gradients along the length of the threads and as precursors in the mussel foot. We discuss a 76-kDa precursor, preCol-NG, from a cDNA library of the foot where it has no gradient but rather is distributed evenly along the distal to proximal axis. A pepsin-resistant fragment of preCol-NG has been confirmed in byssal threads. Like preCol-D and -P, this protein has a central collagenous domain, flanking domains, an acidic patch, and histidine-rich termini. The flanking domains of preCol-NG resemble the glycine-rich proteins of plant cell walls with tandem XGlyn repeats where X denotes alanine, leucine, or asparagine but not proline. Similarity with the (glycine–alanine) repeats and poly(alanine) runs of arthropod silks also exists. Based on available evidence, a model of preCol axial assembly is proposed in which preCol-NG functions as a mediator between preCol-D/-P molecules. This is consistent with the observed progression of mechanical properties in byssal threads.

Molecular and Biochemical Characterization of Cytosolic Phosphoglucomutase in Maize1 : Expression during Development and in Response to Oxygen Deprivation

Phosphoglucomutase (PGM) catalyzes the interconversion of glucose (Glc)-1- and Glc-6-phosphate in the synthesis and consumption of sucrose. We isolated two maize (Zea mays L.) cDNAs that encode PGM with 98.5% identity in their deduced amino acid sequence. Southern-blot analysis with genomic DNA from lines with different Pgm1 and Pgm2 genotypes suggested that the cDNAs encode the two known cytosolic PGM isozymes, PGM1 and PGM2. The cytosolic PGMs of maize are distinct from a plastidic PGM of spinach (Spinacia oleracea). The deduced amino acid sequences of the cytosolic PGMs contain the conserved phosphate-transfer catalytic center and the metal-ion-binding site of known prokaryotic and eukaryotic PGMs. PGM mRNA was detectable by RNA-blot analysis in all tissues and organs examined except silk. A reduction in PGM mRNA accumulation was detected in roots deprived of O2 for 24 h, along with reduced synthesis of a PGM identified as a 67-kD phosphoprotein on two-dimensional gels. Therefore, PGM is not one of the so-called “anaerobic polypeptides.” Nevertheless, the specific activity of PGM was not significantly affected in roots deprived of O2 for 24 h. We propose that PGM is a stable protein and that existing levels are sufficient to maintain the flux of Glc-1-phosphate into glycolysis under O2 deprivation.

Quantitative trait loci and metabolic pathways: genetic control of the concentration of maysin, a corn earworm resistance factor, in maize silks.

Interpretation of quantitative trait locus (QTL) studies of agronomic traits is limited by lack of knowledge of biochemical pathways leading to trait expression. To more fully elucidate the biological significance of detected QTL, we chose a trait that is the product of a well-characterized pathway, namely the concentration of maysin, a C-glycosyl flavone, in silks of maize, Zea mays L. Maysin is a host-plant resistance factor against the corn earworm, Helicoverpa zea (Boddie). We determined silk maysin concentrations and restriction fragment length polymorphism genotypes at flavonoid pathway loci or linked markers for 285 F2 plants derived from the cross of lines GT114 and GT119. Single-factor analysis of variance indicated that the p1 region on chromosome 1 accounted for 58.0% of the phenotypic variance and showed additive gene action. The p1 locus is a transcription activator for portions of the flavonoid pathway. A second QTL, represented by marker umc 105a near the brown pericarp1 locus on chromosome 9, accounted for 10.8% of the variance. Gene action of this region was dominant for low maysin, but was only expressed in the presence of a functional p1 allele. The model explaining the greatest proportion of phenotypic variance (75.9%) included p1, umc105a, umc166b (chromosome 1), r1 (chromosome 10), and two epistatic interaction terms, p1 x umc105a and p1 x r1. Our results provide evidence that regulatory loci have a central role and that there is a complex interplay among different branches of the flavonoid pathway in the expression of this trait.